1. Field
The invention pertains to cleaning water and oxygen impurities from hydrogen gas which is to be stored in tanks containing a hydride-forming metallic alloy. The water and oxygen impurities, if left in the hydrogen, would have a detrimental effect on the hydride-forming metallic alloys in the storage tanks.
2. State of the Art
Because of the abundance of hydrogen and its relatively pollution-free burning qualities, the desirability of developing hydrogen as an energy source has long been recognized. A major obstacle or drawback in utilizing hydrogen thus far has been the difficulty of efficiently and safely storing hydrogen. Storing hydrogen as a liquid is costly since it requires considerable energy to liquify the hydrogen, and transfer of the liquid from one container to another results in a loss to the atmosphere of much of the hydrogen. Also, containers for the liquid hydrogen must be extremely well insulated to reduce the loss of hydrogen due to vaporization or boiling. Storing hydrogen as a gas requires extremely heavy and bulky containers and is impractical for most presently contemplated uses.
The use of hydride-forming metallic reactant (hereinafter defined to mean any metals, metal compounds or alloys reacting with and thereby capable of absorbing hydrogen) appears to be an attractive approach to the storage of hydrogen. Exemplary hydride-forming metallic reactants include alloys comprising at least two elements selected from the group consisting of iron, titanium, nickel, calcium, magnesium, manganese, and rare earth elements. Particularly advantageous alloys include iron-titanium, lanthanum-nickel, calcium-nickel, manganese-nickel, mischmetal-nickel, and mischmetal-calcium-nickel alloys. Storage of hydrogen in the hydride-forming reactant, i.e., forming hydrides in a process which is sometimes referred to as hydriding, typically involves applying hydrogen gas under pressure of from about 150 to 1,000 psia to the material while dissipating the heat generated by the hydride-forming reaction. After the reactant reacts with and absorbs the hydrogen, the container is sealed under pressure to maintain the reactant in the "hydrided" state until the hydrogen is needed for subsequent use. Discharging hydrogen from the storage tanks involves a process substantially opposite that used for storing the hydrogen, i.e., releasing some of the pressure on the tank in which the hydride is contained. The discharge rate of hydrogen can be increased by heating the hydride in the tank.
Hydride-forming reactants presently contemplated for use in storing hydrogen not only react with and absorb hydrogen but also react with and absorb water vapor and oxygen, which are generally present with commercial sources of hydrogen. These impurity gases form much more stable bonds with the metallic reactant than does hydrogen, and whereas hydrogen can be regenerated by lowering the pressure and/or heating the reactant, oxygen and water cannot. Ultimately, the reactant will react with and absorb sufficient oxygen and water during successive cycles of storing hydrogen containing such impurities, that the reactant becomes unsuitable for storing hydrogen.
3. Objectives
One of the principal objects of the present invention was to provide an efficient system for purifying hydrogen gas of oxygen and water prior to charging the hydrogen gas to storage tanks containing a hydride-forming metallic, reactant. A further object of the invention was to provide a system which was self-regenerating, i.e., that upon discharge of hydrogen from the storage tank, the system would regenerate or reactivate itself so as to be capable of removing oxygen and water impurities from succeeding charges of hydrogen containing such impurities.